The present invention relates to fasteners in general and in particular to locking systems comprising a nut and a bolt which lock together when tightened against a workpiece.
Many applications, such as encountered in shipbuilding and aircraft and railroad car manufacturing, require fasteners that have high locking forces. It is, therefore, critical that the nut and bolt assembly hold together under heavy loads and working conditions in which the assembly is subjected to vibrational and other fluctuating stresses. Typically, in such manufacturing processes, pre-set powered torque wrenches are employed on assembly lines to sequentially apply large quantities of lock nuts to corresponding bolts. The lock nuts are tightened to a predetermined torque.
Various devices have been employed in the past to lock a nut and bolt together. One method is to deform the threads of the nut so that they bear in radial compression against the threads of the pin. This deformation may occur before or after the nut is set. One example is shown in U.S. Pat. No. 2,940,495 issued to G. S. Wing, which discloses a lock nut with a frangible drive portion that is sheared off after a predetermined torque is reached, leaving a portion of the fastener tightened onto the bolt with a predetermined and known torque. In manufacturing, the nut is distorted out of round by application of force on diametrically opposed points, producing an elliptical cross section. Once the nut is set, the deformed threads of the nut bear in radial compression against the threads of the pin. The resistance to unthreading is purely frictional.
Such fasteners have several disadvantages. If the nut is deformed prior to assembly with the bolt, the threading operation is more difficult due to the out-of-round configuration of the nut. If the deformation occurs after the nut is set, an additional field step is required to effect the deformation. Moreover, the holding force generated by the deformation of the nut is purely frictional. In the Wing fastener, the fact that a section of the nut is sheared off during application creates a multitude of scrap pieces which must be removed from the environment.
Another approach is disclosed in U.S. Pat. No. 4,260,005, issued to Stencel. Stencel discloses a nut or collar having at least one lobe extenting radially outward on the outside of the collar. The lobe has an external surface for engagement by the driver. As the collar is threaded onto the bolt or pin, resistance to threading increases. At a predetermined load, the lobe deforms radially inward and diplaces material of the body ahead of the deforming lobe into the axial bore of the collar, thereby locking the collar and pin together. The pin has a plurality of axially extending flat surfaces of smaller diameter than the major diameter of the pin, which provide void volumes for receiving the deformed material of the collar.
The Stencel device also suffers from a number of disadvantages. The lobes of the Stencel collar abruptly change the outer configuration of the collar. As a result, the lobes can shear from the collar with a sufficient tangential component of force. Thus, the Stencel disclosure warns that the relationship between the driver and lobes during the development of the lock should be one that produces radial failure of the collar in the zone of the lobes and not shearing of the lobes from the collar. Secondly, the lobes make it difficult to register an automatic power torque wrench which is rotating at high revolutions per minute upon contact with the collar. Finally, the tooling for the collar is relatively expensive.
As a result, there remains a need for a self-locking fastener system in which a nut can be applied to a bolt that will lock together after a predetermined torque is reached, effectively, efficiently, without waste of material and without expensive tooling.